Freshwater systems have been altered since historical times, but such modifications skyrocketed in the early to mid-1990s. Projects include:
- modifying waterways to improve navigation,
- draining wetlands,
- constructing dams and irrigation channels, and
- establishing interbasin connections and water transfers.
These changes have improved transportation, provided flood control and hydropower, and boosted agricultural output by making more land and irrigation water available. At the same time, these physical changes in the hydrological cycle:
- disconnect rivers from their floodplains and wetlands and slow water velocity in riverine systems, converting them to a chain of reservoirs,
- impacts the migratory patterns of fish species and the composition of riparian habitat,
- opens up paths for exotic species,
- changes coastal ecosystems, and
- contributes to an overall loss of freshwater biodiversity and inland fishery resources.
The following summarizes key findings of the PAGE study regarding the condition of freshwater systems, as well as the quality and availability of data.
Conditions and trends
- Although water in rivers, lakes, and wetlands contains only 0.01 percent of the world’s freshwater and occupies less than 1 percent of the Earth’s surface, the global value of freshwater services is estimated in the trillions of U.S. dollars.
- Dams have a significant impact on freshwater ecosystems. Large dams have increased sevenfold in number since 1950 and now impound 14 percent of the world’s runoff.
- Sixty percent of the largest 227 rivers of the world are strongly or moderately fragmented by dams, diversions, and canals. In all, strongly or moderately fragmented systems account for nearly 90 percent of the total water volume flowing through these rivers.
- In the developing world, large dams are still being built at a fast rate, threatening the integrity of some of the remaining free-flowing rivers in the world. The basins with the greatest number of large dams currently under construction are the Yangtze, the Tigris and Euphrates, and the Danube.
- According to estimates by Vörösmarty et al. (1997a and 1997b), the average residence time of river water in regulated basins has tripled to over one month worldwide, whereas large reservoirs trap 30 percent of the global suspended sediments.
- Half the world’s wetlands are estimated to have been lost during the 20th century, as land was converted to agriculture and urban use, or filled to combat diseases, such as malaria.
- At least 1.5 billion people rely on groundwater as their only source of drinking water. Overexploitation and pollution in many regions of the world are threatening groundwater supplies, but comprehensive data on the quality and quantity of this resource are not available at the global level.
Information status and needs
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Global information on dams and reservoirs is limited to dams that are 15 meters in height or greater, except for China, Japan, India, and Spain, which report only on dams over 30 meters. The largest data gaps are for Russia, which reports only on hydropower dams, and China, where the majority of the world’s large dams have been built and for which information is exceedingly difficult to acquire.
- The provision of latitude and longitude for each dam would highly improve our ability to locate these structures within the correct hydrological unit and assess their downstream impacts.
- Information on discharges is also lacking for many reservoirs; however, these data are needed to assess more fully the annual variations in river flow.
- Another important data set needed to assess freshwater ecosystem conditions is complete global information on wetlands distribution and change. Location and size of wetlands is especially needed for Asia, Africa, South America, the Pacific Islands, and Australia.
- Regional data for Oceania, Asia, Africa, eastern Europe, and the Neotropics allow for only cursory assessment of wetlands extent and location. Only North America and western Europe have better data and monitoring programs in place to track changes in wetlands area.
- Remote sensing data from the new Landsat 7 satellite and from radar, which can sense flooding underneath vegetation and can penetrate cloud cover, should improve the information base on the extent, location, and change in wetlands.
- Limited information is available on groundwater exploitation at the global level. National-level data exist but are not readily accessible or are not harmonized among countries. Groundwater information should be collected in coordination with data collection efforts on the effects of their use on other regional water resources, such as wetlands, lagoons, and river basins.
Quality and availability of data
PAGE measures and indicators
Data sources and comments
Historical alteration of freshwater systems worldwide Compilation of data from the following sources:
- Based on Naiman, R. J., J. J. Magnuson, D. M. McKnight, and J. A. Stanford, eds. 1995. The Freshwater Imperative: A Research Agenda. Washington, DC: Island Press as adapted from L’vovich, M. I. and G. F. White. 1990. “Use and Transformation of Terrestrial Water Systems,” pp. 235–252 in The Earth as Transformed by Human Actions: Global and Regional Changes in the Biosphere Over the Past 300 Years. B. L. Turner II, W. C. Clark, R. W. Kates, J. F. Richards, J. T. Mathews, and W. B. Meyer, eds. Cambridge, U. K.: Cambridge University Press.
- Additional data from
- Shiklomanov, I.A. 1997. Comprehensive Assessment of the Freshwater Resources of the World: Assessment of Water Resources and Water Availability in the World. Stockholm, Sweden: World Meteorological Organization and Stockholm Environment Institute
- ICOLD (International Commission on Large Dams). 1998. World Register of Dams 1998. Paris, France: ICOLD
- Avakyan, A. B. and V. B. Iakovleva. 1998. “Status of Global Reservoirs: The Positionin the Late Twentieth Century.” Lakes & Reservoirs: Research and Management 3: 45–52
- IJHD (International Journal of Hydropower and Dams). 1998. 1998 World Atlas and Industry Guide. Surrey, U. K.: Aqua-Media International
River channel fragmentation and flow regulation Analysis was done by Rivers assessed are those with a historical record of more than 350 virgin mean annual discharge. Based on available information on dams and other flow regulations. Not all regions of the world were assessed.
Nilsson, C., M. Svedmark, P. Hansson, S. Xiong and K. Berggren. 2000. River fragmentation and flow regulation analysis. Unpublished data. Umeĺ, Sweden: Landscape Ecology, Umeĺ University.
Additional data and analysis from
Dynesius, M. and C. Nilsson. 1994. “Fragmentation and Flow Regulation of River Systems in the Northern Third of the World.” Science 266: 753–762.
Number of large dams under construction by river basin IJHD (International Journal of Hydropower and Dams). 1998. 1998 World Atlas and Industry Guide. Surrey, U. K.: Aqua-Media International.
This data set includes only reported dams over 60 meters high that are currently under construction, aggregated by river basin.
Residence time of continental runoff by river basin Vörösmarty, C. J., K. P. Sharma, B. M. Fekete, A. H. Copeland, J. Holden, J. Marble, and J. A. Lough. 1997a. “The Storage and Aging of Continental Runoff in Large Reservoir Systems of the World.” Ambio 26(4): 210–219.
This indicator is based on the analysis of 622 of the largest reservoirs in the world (storage capacity at least 0.5 km3). The residence time of otherwise free flowing water is termed by the authors “aging of continental freshwater.”
Exploitation of groundwater resources Compilation of data and case studies are from the following sources:
EEA (European Environment Agency).1995. Europe’s Environment: The Dobríš Assessment. D.Stanners and P. Bourdeau, eds. Copenhagen, Denmark: European Environment Agency.
British Geological Survey. 1996. Characterisation and Assessment of Groundwater Quality Concerns in Asia-Pacific Region: The Aquifers of the Asia-Pacific Region–An Invaluable but Fragile Resource. UNEP/DEIA/AR.96-1. Prepared on the behalf of the U.N. Environment Programme (UNEP) and the World Health Organization. Nairobi, Kenya: UNEP.
Foster, S., A. Lawrence, and B. Morris. 1998. Groundwater in Urban Development: Assessing Management Needs and Formulating Policy Strategies. World Bank Technical Paper No. 390. Washington, DC: The World Bank.
Scheidleder, J. Grath, G. Winkler, U. Stark, C. Koreimann, and C. Gmeiner. 1999. Groundwater Quality and Quantity in Europe. S. Nixon, ed. European Topic Centre on Inland Waters. Copenhagen, Denmark: European Environment Agency.
Wetlands extent and change in the United States and estimates for some European countries Data for the United States are from the National Wetlands Inventory, U.S. Fish and Wildlife Service (USFWS), and the Natural Resource Inventory of the U.S. Department of Agriculture. European data are from the European Environment Agency. Percentage of cropland and urban and industrial land use by river basin Cropland area is estimated from the at l-km resolution. Cropland in this analysis excludes areas of mixed natural/cropland vegetation. Urban/industrial areas are based on
GLCCD (Global Land Cover Characteristics Database), Version 1.2. 1998. Loveland, T.R., B.C. Reed, J.F. Brown, D.O. Ohlen, Z. Zhu, L. Yang, and J. Merchant. 2000. “Development of a Global Land Cover Characteristics Database and IGBP DISCover from 1-km AVHRR data.” International Journal of Remote Sensing
NOAA-NGDC (National Oceanic and Atmospheric Administration-National Geophysical Data Center). 1998. Stable Lights and Radiance Calibrated Lights of the World CD-ROM. View Nighttime Lights of the World database available on-line at: http://spidr.ngdc.noaa.gov/. Boulder, Colorado, U.S.A.: NOAA-NGDC.
The data set contains the locations of stable lights, including frequently observed light sources, such as gas flares at oil drilling sites. Data were collected in 1994-95.
